1. Field of the Invention
The present invention relates to a resonant inverter and resonance power supply apparatus capable of easily reducing an operating voltage of a main switch.
2. Description of the Related Art
U.S. Pat. No. 7,924,580 discloses a push-pull resonant inverter having a plurality of resonant frequencies and capable of driving switches with a phase difference of 180°. The inverter disclosed in this patent literature is a push-pull inverter having resonance such that two single-ended EF2 inverter sections coupled together with a shared ground are coupled to at least one load. This patent literature also discloses the use of coupling that enables high frequencies in the push-pull sections to be independently tuned. A configuration is also disclosed in which a resonant frequency of a first harmonic resonator and a second harmonic resonator in a section 1 and a resonant frequency of a first harmonic resonator and a second harmonic resonator in a section 2 are individually controlled and some of resonant elements in the second harmonic resonator in each section is shared. A result of the configuration is also disclosed in which an impedance seen by the switching element in each section is the same as an impedance seen by the single-ended switching element and the waveform in the switch in each section is the same as the waveform in the single-ended switch. The necessity of a low impedance at the second harmonic of the driving frequency in a class EF2 operation and production of that impedance by the second harmonic resonator in each section are disclosed.
The disclosed push-pull inverter includes two single-ended inverters coupled together and allows the use of electric power simply twice that in a single-ended inverter. Generally, for high-frequency inverters, driving high-side switches, such as bridge circuits, at high frequencies needs to ensure a short dead time, and this is difficult in terms of prevention of flow-through currents in the high-side and low-side switches. In contrast, for the disclosed push-pull inverter, because only a low-side switch is driven, it is an effective approach to extending an electric power range of an inverter driven at high frequencies.
However, the known push-pull resonant inverter disclosed in the above-mentioned patent literature needs to adjust a resonant frequency in the second harmonic resonator for each inverter section. If the resonant frequency in each section is displaced, waveforms in the sections may become imbalanced, a transformer may be saturated or an operating voltage of the switches may increase, and this may adversely affect the life of the switches. Although the above-mentioned patent literature discloses setting the resonant frequency in the second harmonic resonator at a value twice the oscillation frequency, setting the resonant frequency in the second harmonic resonator at the value twice the oscillation frequency does not necessarily lead to minimization of the operating voltage in the switches, and in consideration of variations or other factors, it may even have an adverse effect on the life of the switches.